A magneto-resistive (MR) element exhibits a change in electrical resistance as a function of external magnetic field. Such property allows MR elements to be used as magnetic field sensors, read heads in magnetic storage systems, and magnetic random-access-memories. In storage systems, the read head reads encoded information from a magnetic storage medium, which is usually a disc coated with a magnetic film. In a read mode, a magnetic bit on the disc modulates the resistance of the MR element as the bit passes below the read head. The change in resistance can be detected by passing a sense current through the MR element and measuring the voltage across the MR element. The resultant signal can be used to recover data from the magnetic storage medium. Depending on the structure of a device, the MR effect can fall into different categories, namely, anisotropic MR (AMR), giant MR (GMR), tunneling MR (TMR), and colossal MR (CMR).
As the areal densities of a hard disc increase beyond about 10 Gbit/inch2, AMR heads give way to GMR heads due to a GMR head's ability to produce a stronger signal. The GMR device currently manufactured in production by the data storage industry is the spin valve. The spin valve consists of a free layer whose magnetization rotates with an external field, a non-magnetic metallic spacer layer (typically a Cu spacer or the like), a reference layer, a thin non-magnetic metallic layer (typically Ru), a pinned layer, and an antiferromagnetic pinning layer, such as PtMn or NiMn. The pinned layer has its magnetization direction fixed by virtue of the interfacial exchange coupling between the magnetization of the pinned layer and the antiferromagnet. The magnetization of the reference layer is in a direction anti-parallel to that of the pinned layer magnetization by virtue of the very strong antiferromagnetic coupling promoted between the pinned and reference layers by the thin non-magnetic metallic layer. In a typical spin valve, the current flows in the plane of the metallic layers. This mode of operation is referred to as current-in-plane (CIP). The electrical resistance of a spin valve is a function of the angle between the magnetization of the free layer and of the reference layer. A sensor exhibits the largest resistance when the two layers are magnetized in antiparallel directions, and the smallest when they are parallel. The technology of GMR read heads has advanced so that it is possible to read from areal densities up to 100 Gbit/inch2, beyond which point the sensitivity and output signal again becomes an issue.
One possible solution to the limitations of spin valve heads is to use tunneling magnetoresistance (TMR) junctions. The typical TMR junction is very similar to a spin valve in the sense that it also includes a free layer, a spacer, and a synthetic antiferromagnet. However, an alternative design may include a spacer layer positioned between two free layers. The free layers are biased by some external source such that their magnetization is aligned in a perpendicular orientation.
A major difference between TMR junctions and spin valves is that in the TMR junction the spacer layer is an oxide or semiconductor barrier as opposed to a conductor. Moreover, the electrical current in a TMR junction flows perpendicular to the plane of the films (CPP mode) as opposed to flowing in the plane for spin valve sensors (CIP mode). Since the spacer layer is an insulator or semiconductor in a tunnel junction, the electrons making up the current travel (“tunnel”) through the barrier from the free layer to the reference layer. The magneto-resistance of a TMR head changes due to the angular difference between the magnetization in the two magnetic layers in a way analogous to a spin valve. However, a TMR signal can be much larger than in spin valves, resulting in more amplitude and sensitivity of the device.
Due to the unique nature of tunneling physics and the CPP current flow, TMR junctions offer more room for engineering design because the TMR signal and resistance are not directly related, as they are in spin valves. In particular, the resistance of the junction depends only on the barrier thickness and junction area and not on the details of the rest of the stack. In theory, the magneto-resistance of a TMR junction depends only on the polarization of the free and reference layers and is independent of the junction area and the details of the other layers in the stack. In practice, for ultra-thin barriers, there is a strong dependence on magnetoresistance as a function of barrier thickness due to processing defects in the barrier. The most common defects are known as pinholes. Pinholes represent very small discontinuities in the tunneling barrier where the free and pinned layers are in direct contact with the tunneling barrier. The current can shunt through these pinholes and reduce the resistance and magnetoresistance of the stack. In spite of defect limitations, the CPP TMR provides tremendous flexibility in head design, allowing independent optimization of the individual layers of the stack. Additionally, the geometry of the head can be optimized to achieve the best head performance without sacrificing head amplitude.
All types of MR elements typically include shields, such as a high permeability film like NiFe. The function of the shields is to protect sensors from the stray magnetic fields originating from all magnetic bits on the medium, except the bit just underneath the sensor. For spin valves, the shields are separated from the sensor by an insulator such as, for example, aluminum oxide or silicon dioxide. The insulator helps to prevent current from leaking into the shields in CIP mode. As the linear bit density of a memory medium track increases, the shield-to-shield spacing of a read sensor must be reduced to adequately screen the flux from adjacent bits.
Spin valves are not well suited for high linear density applications because the shield-to-shield spacing of a spin valve includes both the stack thickness and the thickness of the gaps. TMR junctions are well suited for very small shield-to-shield spacing since the current flows in CPP mode, thus eliminating the need for an insulator between the shields and the sensor. If the shields are used as electrodes to the tunnel junction, the shield-to-shield spacing consists of only the stack thickness. Therefore, tunnel junctions may be superior in some respects to spin valves for high linear density applications.
A further consideration when designing MR elements is the physical line width density in memory media. As the line width (track) density increases, the width of the sensor layers also becomes an issue as does shielding of stray magnetic fields originating from adjacent tracks.
Another issue related to sensor design is the effective resistance of the MR elements. MR system designs typically have a fixed upper limit on the amount of effective resistance the system can tolerate, both from a system perspective and frequency response. In TMR elements, the effective resistance increases as the thickness of the tunnel barrier increases and the area of the TMR junction decreases. Consequently, the standard approach is to make the barrier as thin as necessary to meet the target resistance for a given TMR area. The area of the TMR is given by the product of the track width and sensor height (typically measured in microns). The track width is fixed by the track density and decreases for increasing areal density. The sensor height is not constrained by medium bit geometry and is adjusted to achieve the optimum stability for the device.
The sensor height is typically about 0.5–1.5 times the track width. For example, a typical design for 105,000 tracks per inch would be a sensor width of 0.12 microns and a stripe height of 0.15 microns. If the target resistance of the sensor is 100Ω, the intrinsic barrier resistance multiplied by the area product (RA product) should be about 1.8 Ω·μm2. This requires the film thickness of the barrier to be in the range of about 4–5 Å (for typical oxide barriers). Such a film thickness poses a tremendous challenge for manufacturability. Furthermore, the reliability of the TMR device is significantly impacted by the RA product, with less reliability associated with smaller RA products (or thinner barrier). Specifically, the electrostatic breakdown, electrostatic discharge (ESD) sensitivity, and thermal reliability are all impacted by the RA product, and a lower RA stack junction (barrier) is more prone to being damaged during the processing, assembling, and operating of the TMR heads. There are serous concerns about whether ultra low RA TMR junctions can meet the strict reliability criteria for use in hard drives produced for magnetic storage applications.
Accordingly, there is a need for a new TMR head structure in which the device resistance can be managed for very small junction area while simultaneously providing sufficient amplitude, signal-to-noise ratio, and reliability for long term operation in hard drives. The present invention provides a solution to this and other limitations, and offers other advantages over the prior art.